The same demographic trends that helped the GOP keep the House will hurt their shot at the presidency. And the trends that propelled Obama to reelection will impede Democrats from retaking the House.

Members of the 113th Congress, many accompanied by family members, take the oath of office in the House of Representatives chamber on Capitol Hill in Washington, Thursday, Jan. 3, 2013. (AP Photo/J. Scott Applewhite)

The House of Representatives is not just divided between the red and the blue. It also fractures along lines of white, black, and brown.

Four-fifths of the House Republicans in the new Congress represent districts in which the white share of the voting-age population exceeds the national average, according to a new NationalJournal analysis. In a near-mirror image, almost two-thirds of House Democrats represent districts in which the minority share of the voting-age population exceeds the national average, the analysis found.

For each party, these stark patterns bring opportunities and challenges. The GOP’s strength in these preponderantly white districts helped sustain its House majority in a year when overwhelming minority support powered President Obama to a comfortable reelection. But the party’s disproportionate reliance on whites also means that few House Republicans have much experience in courting nonwhite voters—or much electoral incentive to do so.

That dynamic will likely make it tougher for the party to formulate an agenda, on issues from immigration to health care, that attracts more of the minority support it will almost certainly need to reclaim the White House in 2016 or beyond. “If we are going to become a national governing party again, the first thing we need to do is accept the reality of America as it is today, and these reapportioned districts work against that completely,” says veteran GOP strategist John Weaver. “While I don’t want to lose House seats, I would much rather be entrusted with governing the country than having a permanent House majority that is out of touch with the rest of the country.”

Conversely, the tilt toward minority districts among Democrats reflects both the party’s gains in heavily diverse areas and its systemic decline in the mostly white districts once represented by centrist Democratic Blue Dogs.

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Those twin trends could make it easier for the party to coalesce around common positions on issues that have long divided it, such as immigration and gun control. But with Republicans holding such a commanding advantage in heavily white districts—which still significantly outnumber the diverse districts—it also means that Democrats will struggle to regain a House majority until changing demography brings more seats within their reach.

Above all, these divergent patterns of support threaten to deepen the national polarization so evident in the standoff over the fiscal cliff. In Congress, as in the presidential race, the two parties are supported by electoral coalitions increasingly divided not only by ideology but also by race. Each side’s congressional caucus is now rooted in places that differ enormously from the other side’s, in their demographic composition, cultural values, and attitudes toward government. It’s becoming more difficult to bridge those differences. “It’s a problem for the country,” says Tom Davis, the former Republican representative from Virginia and chairman of the National Republican Congressional Committee. “You hate to have any society ethnically divided. But that’s what we are becoming.”

RACE AGAINST HISTORY

Compared with the wave elections of 2006, 2008, and 2010, the 2012 campaign produced only modest change in the House’s partisan balance, with Democrats adding eight seats. But beneath the surface, a demographic riptide pushed the two parties further apart. Despite their losses, Republicans increased their share of districts that are whiter than the national average; the Democratic gains came entirely from districts that lean toward minorities.

For this analysis, NationalJournal used 2010 census data to rank the 435 House districts based on the share of their voting-age population represented by whites. NJ then compared the districts with the national white share of the voting-age population—which stands at almost exactly two-thirds, or 67 percent, according to the latest census calculations. The voting-age population figure includes residents who are not citizens or are in the U.S. illegally, so it does not measure the share of eligible voters in each district. But it still provides a useful gauge of the demographic bent in the House’s districts—and a clear dividing line in the fortunes of each party.

In districts where the white share of the voting-age population exceeds the national average, Republicans in November captured nine Democratic-held seats and lost seven of their own, for a net gain of two. In seats where the minority share of the voting-age population exceeds the national average, Democrats gained 11 and lost just one, for a net gain of 10. (The calculations are somewhat complicated by the redistricting that occurred after the 2010 census; this tally allocated new seats created by redistricting to one or the other party based on the district’s Partisan Voting Index, a measure of political leaning calculated by The Cook Political Report.)